The invention relates generally to processes and compositions for electrographic image development or powder deposition. More specifically, the invention relates to compositions and methods for electrographic image development, wherein certain relationships between toner and carrier particle sizes, charges, masses and velocity are optimized for sufficient toner kinetic energy in relation to toner potential energy to allow development to completion. The prior art does not describe optimum relationships between toner and carrier properties such as particle size, particle mass, particle charge or velocity. Accordingly, the optimum relationships for toner and carrier properties have, to date, not been determined taking into account toner kinetic energy or toner potential energy and development to completion.
Processes for developing electrographic images with a magnetic brush using dry toner are well known in the art and are used in many electrographic printers and copiers. Toner application processes utilizing a magnetic brush are also being investigated for powder deposition in powder coating systems. These application processes are elucidated in, for example, L. B. Schein, “Electrography and, Development Physics”, Laplacian Press, 1996, the disclosure of which is incorporated herein by reference. The term “electrographic printer,” is intended to encompass electrophotographic printers and copiers that employ a photoconductor element, as well as ionographic printers and copiers that do not rely upon a photoconductor, and powder coating devices that deposit toner onto a biased or electrostatically charged receiver. Electrographic printers typically employ a developer having two or more components, consisting of resinous, pigmented toner particles, magnetic carrier particles and other components. The developer is moved into proximity with an electrostatic image carried on a receiver, whereupon the toner component of the developer is deposited on the receiver, Deposition of toner onto the receiver is driven by the electric field between the electrostatic image on the receiver and the magnetic brush. In electrophotographic printers the receiver is a photoconductor and the toner is subsequently transferred to a sheet of paper to create the final image. Developer is moved into proximity with the imaging member by an electrically-biased, conductive toning shell, often a roller that may be rotated co-currently with the imaging member, such that the opposing surfaces of the imaging member and toning shell travel in the same direction. Located adjacent the toning shell is a multipole magnetic core, having a plurality of magnets, that may be fixed relative to the toning shell or that may rotate, usually in the opposite direction of the toning shell.
The developer is deposited on the toning shell and the toning shell rotates the developer into proximity with the imaging member, at a location where the imaging member and the toning shell are in closest proximity, referred to as the “toning nip.” In the toning nip, the magnetic carrier component of the developer forms a magnetic brush with a “nap,” similar in appearance to the nap of a fabric, on the toning shell, because the magnetic particles form chains of particles that rise vertically from the surface of the toning shell in the direction of the magnetic field. The nap height is maximum when the magnetic field from either a north or south pole is perpendicular to the toning shell. Adjacent magnets in the magnetic core have opposite polarity and, therefore, as the magnetic core rotates, the magnetic field also rotates from perpendicular to the toning shell to parallel to the toning shell. When the magnetic field is parallel to the toning shell, the chains collapse onto the surface of the toning shell and, as the magnetic field again rotates toward perpendicular to the toning shell, the chains also rotate toward perpendicular again. Thus, the carrier chains appear to flip end over end and “walk” on the surface of the toning shell and, when the magnetic core rotates in the opposite direction of the toning shell, the chains walk in the direction of imaging member travel.
As the carrier chains move in response to the magnetic field, toner particles attracted to the carrier particles by electrostatic interactions are drawn along with the carrier particles. Electrographic printing is based on the electrostatic Coulomb force, FCoul, and the force from the electric field of image development, qE. For toner of charge QT=q, and carrier of charge QC=−q, the Coulomb force component of the attractive force between a charged toner particle on the surface of a charged carrier particle is proportional to:FCoul=−q2/(RC+RT+s)2,   (1) where RC is the radius of the carrier particle, RT is the radius of the toner particle, and s is the separation between the particle surfaces. The prior art commonly models toner particles as spheres with uniform charge distributions, thus allowing a toner particle of total charge q and approximate radius RT to be represented as a point charge q at the center of a sphere. As toner size is increased or decreased, for tribocharged toners, the total charge usually increases or decreases proportionally with the surface area. In other words, for a given toner and carrier formulation, the charge density per unit area is approximately constant as toner size is changed.
Other attractive forces involved in toner-carrier interactions include charge induced polarization, i.e., forces arising from an electrostatic image charge induced in the carrier particle by an adjacent toner particle and, likewise, from polarization induced in the toner particle by an adjacent carrier particle. Additionally, field induced polarization arises from the polarization of each particle in the external electric field of image development. Furthermore, non-uniform charge distributions on toner particles resulting from unequal distribution of charge arising from tribocharging have recently been found to be a major component of the binding force. Finally, dispersion forces or Van der Waals forces act to bind the toner particle to the carrier particle, as well as capillary forces due to adsorbed films of water on toner and carrier surfaces. These forces are referred to as adhesion forces Fa, and generally act over short ranges and are nearly the same magnitude or greater than the Coulomb force FCoul binding toner to the carrier at very short separation distances. Additional forces on toner include impact forces due to collisions and viscous drag during motion of toner through air. These are also of approximately the same magnitude as the Coulomb force binding the toner to the carrier.
For toning to occur, according to the prior art, the force from the electrographic image qE plus the force from the electrographic image carried on the imaging member must be greater than q2/(RC+RT)2 plus the force due to image charges in the carrier and other attractive forces on the toner. The prior art disclosed in Schein describes several theories of development. First, the “neutralization theory” assumes that development occurs until the toner charge per unit area completely neutralizes the imaging member charge. Second, the “field stripping” theory assumes that toner is stripped from the carrier when the qE force due to the electric field of the image overcomes the total force of adhesion of toner to carrier. Third, the “powder cloud” theory assumes that a cloud of toner is produced in the development region and then collected by the electric field associated with the image. Powder cloud development was originally associated with cascade toning and was extended to magnetic brush development. Several authors have concluded that this is a small component of development compared to the principle magnetic brush development process and that it possibly contributes to background toning.
Finally, the “equilibrium” theory holds that toner develops only in three-body contact events in which the toner simultaneously contacts both the carrier and the electrographic imaging member, and toner adhesion forces to the carrier are balanced by toner adhesion to the imaging member. The equilibrium theory includes aspects of the field stripping model, surface forces, and residual charge on the carrier particles. It is the most widely accepted model of toning at the present time. All descriptions of the equilibrium theory incorporate the same forces that would be present if the developer and imaging member were stationary, rather than in motion relative to each other.
There are three versions of the equilibrium theory, differing in their respective prediction of when toning stops. In the first variant of the equilibrium theory, toning is predicted to stop when the forces on the nth toning particle from the carrier are equal and opposite to forces from the imaging member. In a second variant of the equilibrium theory, toning is predicted to stop when the electric field in the air gap between the separated toner and carrier vanishes. Finally, in the third variant, toning is predicted to stop when the total charges in a volume (Gaussian pillbox) with one end in the air gap outside of the toner layer on the image and the other end in the imaging member ground plane equals zero.
All of the foregoing theories take into account only forces that would be the same if the imaging member or developer were not moving. None of these prior art models explicitly incorporates the carrier velocity perpendicular to the imaging member surface or the properties of the collisions of toner or carrier with the imaging member to explain when development will occur.
Accordingly, there is a need in the art for an electrographic image development process that incorporates optimum relationships between toner and carrier characteristics and properties, taking into account the kinetic energy of the developer particles as they move perpendicular to the imaging member in response to magnetic pole transitions resulting from the action of a rotating magnetic field vector.